Two-stage process of surface-hardening workpieces of hardenable ferrous alloys

ABSTRACT

AN ELECTRIC CIRCUIT IS OPERATED AT A STABILIZED, HIGH FREQUENCY. A COLD PLASMA IS PRODUCED, WHICH CLOSES SAID CIRCUIT. ENERGY IS SUPPLIED BY SAID PLASMA AT A UNIFORM RATE TO SUCCESSIVE ADJOINING ELEMEMTAL SURFACE AREAS OF A WORKPIECE OF A HARDENABLE FERROUS ALLOY TO FORM IN SAID ELEMENTAL SURFACE AREAS METASABLE AUSTENITIC PORTIONS HAVING CONSTANT CROSS-SECTIONAL SHAPES AND CONSTANT PROPERTIES.

p 1974 K. SWOBODA EI'AL 3,

TWSTAGE.PROCESS OF SURFACE-HARDENING WORKPIECES- OF HARDENABLE FERROUS ALLOYS 'Filed Sept. 21, 1971 Fig?!" N N N 3,834,947 TWO-STAGE PROCESS OF SURFACE-HARDENING WORKPIECES OF HARDENABLE FERROUS ALLOYS Karl Swoboda, Vienna, and Alfred Kulmburg and Rupert Bltich, Kapfenberg, Austria, assignors to Gebr. Bohler & Co. Aktiengesellschaft, Kapfenberg, Austria Filed Sept. 21, 1971, Ser. No. 182,420 Claims priority, application Austria, Sept. 21, 1970, A 8,499/70 Int. Cl. B23k 9/00; C21d 7/04 US. Cl. 148-12 2 Claims ABSTRACT OF THE DISCLOSURE An electric circuit is operated at a stabilized, high frequency. A cold plasma is produced, which closes said circuit. Energy is supplied by said plasma at a uniform rate to succesive adjoining elemental surface areas of a workpiece of a hardenable ferrous alloy to form in said elemental surface areas metastable austenitic portions having constant cross-sectional shapes and constant properties.

The present invention relates to a process of surfacehardening workpieces of hardenable alloys of iron and steel, particularly a process which enables on the surface of the workpiece a formation of areas of metastable austenite, which have highly constant properties even after a very long treatment.

The production of metastable austenite is the most important result of a two-stage surface-hardening process. In that process, only small elemental areas of the surface of the workpiece are austenitized in succession in very short periods in the first stage so that the material around each elemental area remains initially cold. Heat flows thereafter from the austenitized area into the interior of the workpiece most rapidly so that the austenite is cooled rapidly and is substantially preserved rather than being transformed into martensite. The austenite thus produced is comparable in its toughness properties to conventional austenite but distinguishes from the latter in that it is harder than the martensite which can be produced by the usual quench hardening process at a given carbon content. This high austenite hardness is due to the fact that in the hardness test the penetrating indenter produces a plastically deformed region, which during its formation is transformed at least in part into an extremely hard martensite. This metastable austenite differs basically from the conventional austenites, which are stable when stressed at room temperature and for this reason exhibit a low hardness in the hardness test. This transformation of the metastable austenite to a higher or lower degree in the second stage takes place not only during the hardness test but when the material is subjected to any stress at room temperature. For this reason, no additional steps are required to ensure the desired behavior in use and the twostage surface-hardening process can be carried out in one operation, in which adjoining small elemental areas are austenitized in succession to produce a hardened strip or a hardened portion, e.g., a hardened cutting edge of a tool. The way in which that operation is performed is decisive for the success of the process and for the quality of the workpiece surface which has been treated.

When the austenitization takes place too rapidly during that operation, the same will result in elemental surface areas which contain a very high proportion of martensite or consist entirely of martensite. Such areas have properties which are comparable to those of white friction martensite layers, which are known to be brittle and to have a tendency to spall. If too much energy is introduced in the austenitizing operation, disturbing fusion effects may United States Patent 3,834,947 Patented Sept. 10, 1974 result on the surface of the workpiece or at the cutting edges of tools and the resulting fused portions must be removed before the workpiece is used.

By metallographic examination, the results of such surface treatments can be recognized as white areas which can be etched only with difliculty and which cannot be resolved microscopically. Intense etching treatments may be used to find out whether these white areas consist of martensite or of metastable austenite and result in the formation of martensite needles when austenite is present adjacent to the surface of the polished section. Such martensite needles cannot be formed if the white areas are martensitic before being etched. White areas can also be distinguished or identified by an X-ray examination to determine the proportion of austenite.

Areas of metastable austenite can be produced on workpieces of alloys of iron or steel if sufficiently high carbon contents can be dissolved within very short time in the austenitizing operation. If carbides are present which cannot be dissolved or can be dissolved only with difiiculty or if segregated graphite is present, the matrix must have a sufiiciently high carbon content and in that case the carbides or graphite are contained in an undissolved state in the white area. The minimum carbon content which is required is about 0.6% with plain carbon steels and may be even lower with alloy steels.

Friction discs, plasma torches and electron beams have been used so far as sources of energy in carrying out the two-stage surface-hardening process. An important requirement to be met by such source of energy resides in that it must enable such a concentration of energy that the required austenitization can be enforced within extremely short times, which amount to less than 10- second, although this figure is suggested by considerations which are merely qualitative.

Another essential requirement resides in that a uniform supply of energy into the workpiece must be enabled throughout the treatment so that the resulting austenitic areas are uniform in form and properties.

Whereas these two requirements can be met in a fairly simple manner with sufiiciently stable electron beams, the same require a treatment of the workpiece in a high vacuum. Because expensive plant is required for this purpose, electron beams cannot be used in mass production and their use is economical only in special cases.

When friction discs are used, which rotate at high peripheral velocities of more than meters per second and suitably consist of hardened high-speed steel and have end faces formed with a suitable contour, energy is supplied in the form of the frictional heat which is generated when such end faces are forced against the workpiece. During this operation, e.g., the workpiece is moved past the disc so that a trace is produced on the workpiece and material is removed from the latter in a thickness up to 0.2 millimeter. When this process is incorporated in a production, that removal of material necessitates the provision of corresponding oversize allowances and the treatment must be carried out before the finish-grinding operation if the workpiece should have exactly controlled dimensions. Besides, the removal of material must be most exactly controlled if reproducible results are to be obtained. Further disadvantages of this process reside in the fact that it can be used only with workpieces having a simple configuration, e.g., with cylindrical workpieces or workpieces having flat surfaces or straight cutting or other edges, and that the dimensions of these workpieces must be controlled most exactly. Small local deviations from predetermined dimensions, of an order of 0.01 millimeter, will result in a reduced or increased removal of material so that the supply of energy is irregular and the resulting austenite layer varies in shape. On the other hand, a plant 3 for producing friction austenite is less expensive than plants for producing austenite by other means.

Plasma torches according to the invention utilize the energy which is released by the recombination of the charge carriers of the plasma on electrically conducting surfaces. Where a hot plasma is used, which is produced by the dissociation of the process gas by means of an electric arc, the areas in which these charge carriers are present are surrounded by a hot stream of gas, which is not dissociated and which supplies undesirably large heat quantities to the workpiece so that the previously formed austenite layers are destroyed by tempering actions and additional layers cannot be formed because the temperature of the workpiece is too high. This remark will be particularly applicable if the treatment is carried out for a relatively long time. For this reason, it has already been proposed to use the cold flame of a high-frequency plasma torch in a surface-hardening process. That flame constitutes a cold plasma, in which paper cannot be ignited. The desired charge recombination can also be effected with a cold plasma on electrically conducting surfaces, such as metallic surfaces, so that an extremely rapid heating can be ensured Without disturbing secondary effects produced by hot gases. A cold plasma has the advantages that it can be used in a normal atmosphere, the plant required has reasonable costs, and the process is independent of the configuration and the dimensional tolerances of the workpieces to be treated.

If a workpiece is treated with a cold plasma and special precautions are not taken, the supply of energy will not be suflicient in general to produce austenite layers. Even more disturbing is the fact that the result of the treatment is so irregular in most cases that a commercially useful surface-hardened surface having an area of a few square centimeters or more cannot be produced. Similar results will be obtained in treating cutting edges of tools. Such treatment may result simultaneously in an excessive supply of energy with local fusion and in inadequate hardness values.

Hence, a process with which these difliculties can be avoided would be of great technological significance.

An essential requirement for a commercial two-stage surface-hardening process using a cold plasma resides in that energy must be supplied to the workpiece at a uniform rate during the treatment. This requirement can be met if, in accordance with the invention, the workpiece to be treated is included in an electric circuit which is operated at a constant, high frequency and which is conductively closed by the cold plasma flame.

The difficulties which have been pointed out cannot be avoided if a high-frequency generator is used which is not frequency-stabilized or if a frequency-stabilized hightrequency generator is used and the workpiece forms during the plasma treatment an open oscillating circuit together with the electrode so that the workpiece acts as a capacitive return conductor.

Hence, the invention provides a two-stage process of surface-hardening workpieces of hardenable alloys of iron and steel, and the invention resides in that a cold plasma is used to supply energy at a uniform rate into the workpiece during the treatment to produce elemental surface areas which consist of metastable austenite and have constant cross-sectional shapes and constant properties and said plasma conductively closes a circuit operated at a constant high frequency.

The invention will be described more fully with reference to the accompanying drawings, in which FIG. 1 shows an arrangementfor carrying out the process according to the invention; and I FIG. 2 shows a plasma torch for use in the process.

The arrangement shown in FIG. 1 comprises a frequency-stabilized high-frequency generator, the output power of which can be infinitely controlled. The arrangement also comprises a torch 2 for producing a cold plasma 3, which is used to treat a workpiece 5, which is grounded at 4 just as the high-frequency generator. The arrangement also comprises a matching unit 6 of conventional design, which is connected between the high-frequency generator and the torch and serves to control the optimum rate at which energy is supplied to the workpiece. That rate can be checked by means of a power meter 7 for measuring the power which is supplied to the workpiece and a power meter 8 for measuring the reflected power.

The high-frequency circuit should be operated at a frequency of at least 10 megacycles per second. It is pres ently believed that the upper frequency limit is 100 megacycles per second. The frequency which is selected must comply with the regulations imposed by the telecommunications authorities. A frequency of 13.56 megacycles is preferred at the present time.

The maximum power of the high-frequency generators should lie between at least 1 and 5 kilowatts and should be infinitely variable so that the useful power supplied to the workpiece can be varied as required. The matching unit is required to ensure that the useful power supplied to the workpiece is as high as possible and the reflected power or power loss is minimized. A measurement of these powers is also required to enable a very simple recording of the energy which has been supplied to the workpiece during the treatment so that complicated and time consuming subsequent checks are not required.

Plasma torches for producing cold plasma are known. They essentially comprise a tube in which the plasma is formed either by a high-frequency electric field established by means of an electrode axially extending in the tube, or by a high-frequency magnetic field established by a high-frequency coil which surrounds the tube. An ignition in the torch results in a liberation of electrons from the process gas. In the high-frequency field, these electrons are subjected to such a high acceleration that they can dissociate and ionize the molecules of the process gas to form a plasma.

The plasma torch shown in FIG. 2 comprises a rodshaped electrode 9, which by a cable 10 can be connected to the source of high-frequency power. The torch also comprises a tube 11, which surrounds the electrode and can receive process gas through a short connecting pipe 12. Further, the torch comprises an electrode holder 13 for holding the electrode in an axial position in the tube, and a nozzle 14 for shaping the plasma flame 15. To ignite the torch, the electrode is touched with a metal or carbon rod, which is secured in an insulator. As the rod is withdrawn, a high-frequency electric arc is formed, which initiates the formation of the plasma flame, which issues from the electrode and is maintained in the normal atmosphere.

The electrode may consist of thoriated tungsten. Commercially available argon as used for welding has proved most satisfactory as a process gas in the two-stage surfacehardening process.

The selection of the material for the torch tube is not critical. That tube may consist of electrically non-conducting material or of metal, e.g., copper, if the process gas flows at such a high velocity as to prevent a flash-over between the electrode and the metal tube. An electrically non-conducting material must be selected for the electrode holder. The torch tube may be subjected to excessively high stresses by the radiant heat from the workpiece. In that case the tube must be protected by suitable means. Such protection may be provided in that the tube is cooled or provided with a nozzle for shaping the plasma flame. The nozzle must consist of electrically non-conducting material and must have an adequate resistance to the radiant heat. Every refractory ceramic composition may be used in practice for this purpose.

EXAMPLES Various workpieces were treated in accordance with the invention with a plant for producing a cold plasma. The

high-frequency generator of that plant was operated at a stabilized frequency of 13.56 megacycles per second and had a maximum power input of 1.25 kilowatts.

(a) The workpiece consisted of a water-hardened plate havin dimensions of 100 x 60 x 14 millimeters and made from plain carbon steel containing 1.1% carbon. That plate was treated to form traces of metastable austenite in a width of 25 millimeters. These traces had in crosssection the shape of a segment of a circle with a depth of 0.3 millimeter. The microhardness of these traces was determined under a load of 100 grams and amounted to 950-1000 kilograms per square millimeter whereas the water-hardened surface had a microhardness of 800 kilograms per square millimeter. The traces were entirely uniform in cross-section throughout their length. The p'ate was moved past the torch at a distance of about 5 millimeters therefrom and at a feed rate of 160 millimeters per minute and the power supplied through the plasma flame to the plate amounted to 500 watts.

(b) The workpiece consisted of a reinforcing bar having a cross-section of 8 x 2 millimeters and a length of 2000 millimeters and consisting of a heat-treated steel containing 06% carbon, 03% silicon, 0.6% Mn and 0.1% Cr. The plate had a microhardness of 360 kilograms per square millimeter. To harden an edge of the plate, a portion of metastable austenite was produced. That portion had the shape of an isosceles triangle in cross-section. The legs of the triangle extending from the edge had a length of 0.5 millimeter. A microhardness of 900950 kilograms per square millimeter was measured in the austenitic portion. The treatment of that reinforcing bar was carried out at a feed rate of 810 millimeters per minute and with a useful power of 300 watts.

(c) The workpieces consisted of various bandsaws for wood having cross-sections of 0.7 x 10 millimeters, 0.7 x 20 millimeters and 0.7 x 25 millimeters and a tooth depth of 2.02.6 millimeters. To harden the tooth tips of said saws, these tooth tips were moved past the plasma torch at a distance of about 5 millimeters and at a feed rate of 810 millimeters per minute. The useful power amounted to 150 watts. After the treatment, the tooth tips of these saws had portions which were triangular in cross-section and consisted of metastable austenite. The austenitic portions had a microhardness between 930 and 1000 kilograms per square millimeter. The saws themselves had a microhardness between 420 and 440 kilograms per square millimeter and consisted of steels containing about 0.7% carbon, 0.3% silicon, 0.6% manganese, 0.6% nickel, 0.2% tungsten, and 0.05% vanadium.

The edge life of saws having tooth tips of metastable austenite was twice to four times the edge life of conventional saws.

What is claimed is:

1. A method of surface hardening hardenable steels which comprises the steps of passing a stream of ionizable gas through an alternating electromagnetic field produced by an electrode connected to the first output of a high frequency power source with a maximum power input between 1 and 5 kilowatts which is operable at a stabilized frequency between 10 and megacycles per second, by which means a cold electrically conducting plasma is produced, bringing successive adjoining elemental surface areas of a workpiece of hardenable ferrous alloy, which workpiece is electrically connected to the second output of said high frequency power source, into contact with said plasma thereby closing an electric circuit, controlling the rate at which energy is supplied to said surface areas by measuring the useful power applied to the workpiece and the power lost therefrom and adjusting both of these quantities to ensure the former is optimal and the latter minimal, whereby the surface areas of the workpiece are rapidly raised to a temperature between the upper transformation point and the melting point of said ferrous alloy by the recombination of opposite sign charge carriers of the plasma to form after rapid dissipation of heat by the workpiece an austenitic layer which is metastable at room temperature, and imparting an effective amount of energy to said metastable layer to transform same into finegrained martensite, whereby the surface is hardened.

2. A process as set forth in claim 1, in which said plasma is produced from commercially available argon as used for welding and in a torch having a rod-shaped electrode.

References Cited UNITED STATES PATENTS 3,158,729 11/1964 Gross 219l21 P 3,218,431 11/1965 Stauffer 219121 EB 3,264,508 8/1966 Lai et a1 219-l21 P 3,271,556 9/1966 Harris 2l9l21 EB 3,353,060 11/1967 Yamamoto et a1. 219-121 P 3,615,924 10/1971 SWObOda et a1 148l43 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

148--4, l3; 2l9-121 P 

